The lowering and raising of the massive structures that are to be lowered to the sea bed or raised from the sea bed to the surface, is difficult because of the mass of said structures or of said shuttle tanks. It is known to lower loads having an apparent weight in water of several hundred (metric) tonnes to the sea bed using hoist means situated on a floating support, e.g. a crane; but when the depth becomes considerable, the use of conventional steel cables is problematic since, in addition to the load of said structure, it must also support its own weight, and that can represent up to 50% of said load capacity for a depth of 3000 meters (m). Synthetic cables can also be used that do not present that drawback, but their cost is very high and their use with winches or capstans presents extreme difficulties for heavy loads and depths of 1000 m to 4000 m, or even greater.
In order to lower such loads, it is advantageous to make them lighter by adding buoyancy elements thereto that reduce their apparent weight in water, consequently requiring hoists of lower capacity.
The term “buoyancy element” refers to an element that presents a dead weight that is lighter than sea water, and that thus makes it possible to increase the overall buoyancy that it forms together with the structure to which it is connected or in which it is integrated.
The term “to increase the buoyancy” of an element when it is immersed refers to increasing the ratio ω between the buoyancy thrust exerted on said element and its dead weight out of water. Thus, if said ratio is ω<1, the element has negative buoyancy, so it tends to sink, if ω=1, said element is in equilibrium, and if ω>1 said element floats and its buoyancy increases as ω increases.
The buoyancy of the structure can be made positive so as to make it easier for said structure to rise. For “positive buoyancy”, said buoyancy elements compensate the weight of said structure, so that the buoyancy thrust that is applied both to said structure and to said buoyancy elements is not less than the dead weight of said structure and said buoyancy elements taken together, with the resultant of the forces being directed upwards for positive buoyancy.
The additional buoyancy is generally achieved by using airtight tanks that are filled with air and secured to said load. Such buoyancy elements constituted by air-filled tanks must be capable of withstanding the maximum immersion pressure without imploding or deforming, since the buoyancy would be reduced correspondingly, or even eliminated. The tank must thus be strong enough to withstand the pressure that corresponds to the envisaged immersion depth, which pressure is about an additional 10 mega pascals (MPa) for each additional 1000 m of water depth. Thus, for very great depths, e.g. greater than 1000 m, the casing of the tank must be reinforced sufficiently to withstand the pressure, and its dead weight is consequently much heavier, thereby reducing the performance of said buoyancy element considerably. In order to limit the effects of water pressure at great depth, the tank is advantageously pressurized before it is lowered, thereby making it possible to reduce the dead weight of the tank, since, at the maximum immersion depth, the pressure difference between the outside and the inside is smaller and the wall needs less strength; however, the tank must be capable of withstanding the initial burst-pressure during pressurization.
In order to create said buoyancy, it is also possible to use liquids that are quasi-incompressible, and that present density that is less than that of sea water, e.g. liquids such as fresh water, gas oil, or methanol, that enable less strong casings to be used. However, those materials do not present a ratio ω (buoyancy thrust/dead weight) that is as great as does air, namely: ω=1.026 for fresh water; ω=1.21 for gas oil; and ω=1.30 for methanol.
In order to create buoyancy at very great depths, it is also conventional to use rigid syntactic foam that is made up of microspheres, generally made of glass and of small diameter, mixed with a binder of the polyurethane or epoxy type. That type of foam is capable of withstanding considerable pressure and presents a ratio ω (buoyancy thrust/dead weight) that is more advantageous, lying in the range ω=1.70 to 2.05 for foams that present density lying in the range 0.6 to 0.5, and that are capable of withstanding depths of 1500 m to 2000 m. For syntactic foams that are capable of withstanding greater depths, their density is greater and the ratio ω thus decreases rapidly. Furthermore, such materials based on syntactic foam are very costly and very difficult to manufacture in large volumes, especially for extreme depths.
Once the load is placed on the sea bed, the buoyancy should generally be eliminated so that said load remains stable. For an air-filled tank, it suffices merely to open the valves so that said tank fills with sea water. For a float having a solid buoyancy material such as syntactic foam, the only solution is to separate it by cutting the connections that connect it to the load, and to raise it to the surface, either in controlled manner, which takes a considerable amount of time, or by allowing it to rise freely without any control, which risks creating accidents with the various ships operating at the surface.
The addition of such buoyancy elements makes it possible to reduce the apparent weight in water of the load, but the mass of said load is thus increased by said buoyancy, and by the “added mass” of water, i.e. the mass of water adjacent to the load that is entrained upwards or downwards during vertical movements. Thus, during lowering, although the apparent weight in water of the load may be very light, the inertial mass to be considered is constituted by the mass of the load itself, plus the mass of the buoyancy elements, plus the “added mass” of water, and this can represent an overall inertial mass of 400 tonnes or 500 tonnes for a load mass of 100 tonnes.
It is generally sought to improve the performance of the buoyancy elements, so as to minimize not only the overall inertial mass, but also the size of said buoyancy elements, so as to limit the effects of underwater currents on the load as a whole.